Communications systems are generally defined in terms of their communications parameters (e.g. data rate, channel bandwidth, modulation type etc.). The communications parameters of communications systems are chosen according to the characteristics of the communications channel and the type of communications required.
In mobile communications systems, the propagation of signals between communicating units are affected by the rapidly varying multipath and fading characteristics of the communications channel. Consequently the communications parameters for such mobile communications systems are chosen to take into account the variations in the gain and delay characteristics of the channel due to multipath and fading effects.
Generally the communications parameters of a communications system are fixed. However, to maximize system performance in a mobile communications system, it is advantageous to continuously optimize the communications parameters of the communications system according to the current state of the rapidly varying communications channel.
In order to continuously optimize the communications parameters of a mobile communications system, accurate, real-time information on the characteristics of the communications channels is gathered. The information is then processed and the communications parameters of the system are optimized according to the current channel characteristics.
In the early 1970s, proposals for determining forward channel sounding information were developed. In a communications system that uses a forward channel sounder, the mobile unit transmits a known tone or sequence in an uplink to a Base Station (BS). BSs are medium to high-power multi-channel two-way radios which are in a fixed location and serve as access points between mobile devices and the rest of a communication system. The base station then processes the information and transmits in a downlink the desired communications parameters to the mobile unit for use in future communications between the two communicating units. For TDD systems, the downlink (DL) RF channel is similar to the uplink (UL) RF channel, and thus can be derived at the BS using an uplink sounding signal.
A known method of increasing communication performance is through antenna “beamforming.” Beamforming is a signal processing technique used with an array of antennas, where the directionality of a radiation pattern is controlled by adjusting the phase of the coordinated antennas. When transmitting a signal, beamforming can be used to increase the gain in the direction the signal is to be sent by adjusting the phase of each antenna to create peaks and nulls in the radiation pattern.
An example of a mobile unit communicating with BSs is a mobile unit with two receive antennas, but only one transmit antenna. The DL channel information is used to properly perform the downlink beamforming of the base station antennas. In TDD systems, UL sounding signals from the single transmit antenna of the mobile unit do not provide the DL channel information from the BS antennas to the non-transmitting but receiving-only antenna(s) of the mobile unit. In FDD systems, the estimated UL channel information from the UL sounding signal differs from the DL channel since the UL and DL are on different carrier frequency.
Therefore a need exists to overcome the problems with the prior art as discussed above.